1. Field of the Invention
The present invention relates generally to a semiconductor laser device. It is specifically directed to an improved semiconductor device which, due to a difference between the refractive index of the waveguide portion and the neighboring region results in optical confinement of the emitted light so that the width of the region through which the electrical current passes can be reduced.
2. Description of the Prior Art
The prior art includes semiconductor laser devices having a ridged waveguide structure in which the width of the current passing region is reduced. Referring to FIG. 7, there is shown a distributed feedback semiconductor laser (DFB laser) in which a ridged waveguide structure is employed. The structure includes a semiconductor substrate 50 of a given conductivity type made, for example, of an n-type GaAs, having a major surface. A semiconductor cladding layer 52 having the same conductivity type as the semiconductor substrate 50 and composed, for example, of an n-type AlGaAs is formed on the major surface of the substrate 50 by means of epitaxial growth. An active semiconductor layer 54 made, for example, of GaAs, is then formed on the cladding layer 52 in the same manner. A semiconductor guiding layer 56 having a conductivity opposite to that of the cladding layer 52 and composed, for example, of a p-type AlGaAs is formed on the active layer 54 in the same manner. A periodically corrugated surface which serves as a diffraction grating 58 is formed on the surface of the guiding layer 56 opposite from the active layer 54. The diffraction grating covers the entire surface of the guiding layer and is composed of corrugations which extend laterally, are triangular in cross section, and have a regular pitch interval. After the diffraction grating 58 is formed on the guiding layer 56, a semiconductor cladding layer 60 of the same conductivity characteristics as the guiding layer 56 and composed, for example, of a p-type AlGaAs is formed on the guiding layer 56 by means of epitaxial growth. Then, a semiconductor cap layer of the same conductivity type as the cladding layer 60 and composed, for example, of a p-type GaAs is formed on the cladding layer 60 in the same manner. An etching process is performed selectively to remove both side regions of the cap layer 62 and the cladding layer 60, leaving a central region thereof extending in the longitudinal direction, and the entire lower region of the cladding layer 60, the cap layer 62 comprising only the central region. The cladding layer 60 has a T-shaped cross section. As a result, a stripe structure is formed by the cap and cladding layers 62 and 60. The surfaces of the removed portions of the cap and cladding layers 62 and 60 are covered with insulation films 64. The top surface of the cap layer 62 and the bottom surface of the substrate 50 are provided with counter electrodes 66 and 68 so as to establish ohmic contacts.
In this type of semiconductor laser device, the flow of current can be restricted to a narrow current passing region. However, this type of device does not provide good optical confinement. In order to achieve good optical confinement, the device must be designed to have about 0.01 difference in the refractive index between the central waveguide region and the neighboring region. This difference depends on the effective thickness of the guiding layer 56 (GL) and the thickness d of the neighboring region of the cladding layer 56. However, it is difficult to obtain the desired difference in refractive index by adjusting the thicknesses since the allowable error is so small. Therefore, a device of this type cannot be produced which has consistently predictable optical confinement characteristics.
There has also been proposed a DFB laser having a waveguide which comprises a narrow strip for obtaining uniform and reproducible optical confinement characteristics. One such device is illustrated in FIG. 8 and includes a semiconductor substrate 70 of a given conductivity type composed, for example, of a n-type GaAs, having a major surface. A semiconductor cladding layer 72 of the same conductivity type as the substrate 70 and composed, for example, of n-type AlGaAs is formed on the major surface of the substrate 70 by means of epitaxial growth. An active semiconductor layer 74 composed, for example, of GaAs is then formed on the cladding layer 72 in the same manner. A semiconductor guiding layer 76 having conductivity characteristics opposite to that of the cladding layer 72 and made, for example, of a p-type AlGaAs is formed on the active layer 74 in the same manner. Then, a corrugated strip 78 having a regular period of repitition and which serves as a diffraction grating is formed on the surface of the guiding layer 76 opposite to the active layer 74. Corrugated strip 78 extends over the central region of the guiding layer 76 in a longitudinal direction. The strip 78 defining the diffraction grating is composed of corrugations having a regular pitch and extending perpendicularly to the longitudinal axis thereof. After the diffraction grating 78 is formed on the guiding layers 76, a semiconductor cladding layer 80 of the same conductivity characteristics as the guiding layer 76 and formed, for example, of a p-type AlGaAs is formed on the guiding layer 76 by way of epitaxial growth. Then, a semiconductor cap layer 82 of the same conductivity characteristics as the cladding layer 80 and composed, for example, of a p-type GaAs is formed on the cladding layer 80 in the same manner. Thereafter, ion implantation is performed by injecting ions such as boron ions or the like from the cap layer 82. High resistance current restricting regions 84 are formed on both sides of the cap layer 82 so as to insulate the sections adjacent to central region extending in the longitudinal direction. A pair of counter electrodes 86 and 88 are provided on the top surface of the cap layer 82 and the bottom surface of substrate 70 to establish ohmic contacts therebetween.
This type of semiconductor laser device effectively achieves good optical confinement due to the differences in the refractive indices of the respective sections thereof. However, the current passing region cannot be made narrow so as to increase the reactive current since there is no mechanism for restricting the flow of current to a well defined area within the cladding layer 80.
The aforementioned disadvantages of the semiconductor laser device having a ridged waveguide structure or the narrow strip can also be observed in conventional Fabry-Pe'rot semiconductor lasers.